Groundwater naturally or anthropogenically enriched in As poses serious health hazards in many parts of the world, including the US. This project builds on the growing realization that the particular high affinity of As for Fe-oxyhydroxides is a key factor regulating dissolved As concentrations in groundwater. The particular affinity of As for Fe-oxyhydroxides has been exploited for recommended value to 10 mug/L in the near future, a better fundamental understanding of different treatment options and the influence of groundwater composition on the efficacy of treatment is needed. Them (I) As mobilization into reducing groundwater: Field material from US Superfund sites at Winthrop, ME and Vineland, NH (Project) and Bangladesh (PROJECT 5) will be used to quantify the exchange of As between the dissolved and the particular phase under oxic and reducing conditions. In a first set of experiments, the potential integrated release of As by reductive dissolution will be determined with hydroxylamine extraction from archived aquifer material covering a range of redox conditions. The spatial patterns of As and FE released by reductive dissolution will be compared to the oxygen distribution in the aquifers. In a second set of experiments, fresh cores of aquifer from selected sites will be incubated to quantify the rate of As and Fe exchange (adsorption or desorption). In a third set of experiments, state-of-the-art surface analytical techniques will be used to characterize arsenic binding at model Fe oxide surfaces under a range of conditions representative of the natural environment. Theme (II) As removal from reducing groundwater: Co-precipitation treatment with ferric chloride is an effective technique commonly used for the removal of arsenic form water. A different approach for removing As from groundwater that is promising but has not yet been fully evaluated relies on the adsorptive properties of Fe filings. This method was found to be very effective but has not yet been fully evaluated relies on the adsorptive properties of Fe filings. This methods was found to be very effective at the Winthrop, ME, Superfund site and will be tested at another site in Vineland, NJ. The mechanisms of As removal by Fe filings will also be probed using surface spectroscopy. Another objective under this theme is to develop practical ways of reducing the scale of As remediation and hereby meet the needs of users of domestic wells in the US and Bangladesh. We will focus on Fe-oxyhydroxide-based treatment using, whenever possible, the natural Fe content of groundwater. The As- removal potential of existing Iron Removal Units in Bangladesh will be explored in collaboration with local government researchers. The potential of filings as an alternative source of Fe for co-precipitation to treat water from domestic cells in the US and Bangladesh will also be examined.